A valve timing control system, which is configured to change an angular phase of a camshaft relative to a timing sprocket by virtue of hydraulic pressure, is generally known. In recent years, there have been proposed and developed various valve timing control systems in which an angular phase of a camshaft relative to a timing sprocket that is configured to rotate in synchronism with rotation of an engine crankshaft is changed by transmitting rotary motion (torque) of an electric motor through a speed reducer to the camshaft, so as to variably control intake-valve timing and exhaust-valve timing.
One such valve timing control system has been disclosed in Japanese Unexamined Patent Application Publication No. 2006-207398 (hereinafter is referred to as “JP2006-207398”), corresponding to U.S. Pat. No. 7,603,223, issued on Oct. 13, 2009. In the valve timing control system disclosed in JP2006-207398, two electric-motor-driven valve timing control devices are mounted respectively on the intake camshaft and the exhaust camshaft.